T The long-term objective of this project is to understand how sensorimotor integration contributes to postural stability in humans. A complete understanding of postural control requires knowledge of how the central nervous system (CNS) integrates body-orientation information from multiple sensory sources to generate appropriate balance corrections in response to changing environmental conditions and how biomechanics influence the use of sensory information for postural control. The central hypothesis of this proposal is that the CNS actively regulates sensory integration to achieve optimal dynamic control of balance in environments that alter the available sensory-orientation information and in conditions where biomechanical factors constrain body motion and alter stability limits. The proposed work has three specific aims. The first aim is to identify the passive, biomechanically related and active, neurally-mediated contributions to the generation of balance corrections required for balance control in the sagittal plane. Passive and active mechanisms will be distinguished from one another based on the timing of body-sway responses evoked by continuous perturbations to balance. The postural control system will also be challenged by artificially delaying the arrival of sensory-orientation information to determine how subjects compensate for delayed sensory feedback. The second aim is to determine the ability of the postural control system to integrate multiple sources of sensory-orientation information. The hypothesis is that sensory integration is achieved by a weighted combination of sensory cues, the weighting factors change as a function of environmental and biomechanical conditions, and the selection of weighting factors is determined by an optimal-control strategy. The third aim is to characterize the sensorimotor integration strategies used for the control of body sway in the frontal plane and to determine how biomechanical constraints due to changes in stance width influence the sensorimotor-integration process. The proposed research relies on a model-based approach to explain and to predict postural behavior in a variety of environments and experimental conditions. Methodology developed in engineering disciplines for system identification will be used extensively. Abnormalities involving the active regulation of sensorimotor integration may be an unrecognized source of instability and falls. A better understanding of these active regulatory processes may lead to new rehabilitative methods and new clinical balance-function tests.